9p: add fid-based variant of v9fs_xattr_set()

... making v9fs_xattr_set() a wrapper for it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/9p/xattr.h

@@ -27,6 +27,8 @@ extern ssize_t v9fs_fid_xattr_get(struct p9_fid *, const char *,
 				  void *, size_t);
 extern ssize_t v9fs_xattr_get(struct dentry *, const char *,
 			      void *, size_t);
+extern int v9fs_fid_xattr_set(struct p9_fid *, const char *,
+			  const void *, size_t, int);
 extern int v9fs_xattr_set(struct dentry *, const char *,
 			  const void *, size_t, int);
 extern ssize_t v9fs_listxattr(struct dentry *, char *, size_t);